The only daft question is the one you don't ask👍

Mark & Skipper your both spot on, i/c is short for internal Combustion, again it's the noise factor and speed, but with the improvement of electric motors, i.e brushless the latter isn't so much a factor. Recently my club at Ethrow Park, already i/c are banned, have had to put a speed restriction on boats sailing there, due to complaints from members and public, of some modelers using excessive speed and harassing local aquatic bird life.

Absolutely right! 👍
Internal Combustion; banned due to noise and oil / fuel pollution. Mainly the latter. Big electrics are also fast and dangerous.
On the lake near me in Munich I have seen more than one completely destroy a scale boat by collision. 🤔
Cheers Doug

Ik not completely sure my self but I think you’re on the right track! I’m pretty sure it roughly means the petrol racing boats that whine and go really fast so disturb all the others and are dangerous!